The use of boric acid or other boron containing materials to crosslink polysaccharides, e.g., guar, is known in the art. See U.S. Pat. No. 3,058,909 to Kern, issued Oct. 16, 1962. Such fluids in Kern were used for fracturing subsurface formations. In producing oil and gas, a well is drilled to a subsurface location where oil and/or gas may be present. Sometimes the formation in which the oil and/or gas are located do not allow the oil and/or gas to escape the formation up the oil well bore. One method to increase the amount of oil and gas produced is to make cracks extending out from the well bore into the subsurface formation. One way to do this is to pump a fluid at high pressures down into the well bore to crack the formation and force the fracturing fluid into those cracks. The fracturing fluid carries sand particles or other types of particles, which are called proppants, to hold the cracks open when the pressure is relieved. See the patent to Kern.
A problem encountered during fracturing and during other well operations is the loss of fluid circulation. For example, during fracturing, there may be encountered areas of high permeability, which is an area like a sponge having large holes through it which allow fluids to flow out of the formation into the well bore easily or from the well bore into the formation easily. As noted above, fracturing is used when fluids will not easily flow in or out of the formation. This is like a sponge with very small holes. When the holes get too small, oil for example will not want to come out of the subsurface formation. As noted above, fracturing is intended to make the holes bigger by forming cracks through the formation or increasing the surface area exposed to the well bore. However, if an area with large holes is encountered, pressure may not be maintained during the fracturing operation because the fluid which is being pumped down into the well bore is going into an area that has high permeability, i.e., larger holes, rather than the subsurface formation to be fractured. Sometimes, creating the cracks in the subsurface formation during fracturing may also cause lost fluid circulation. See U.S. Pat. No. 3,198,252 to Walker et al. at col. 1, lines 33-56, and col. 8, lines 36-37 ("F!racturing creates additional fissures and thus aggravates lost circulation problems.").
Another problem encountered with fracturing operations is that the temperature increases the further down into the earth you go. In order to carry the sand particles used to keep the cracks in the subsurface formation open once they are fractured, the fracturing fluid needs to be able to carry these particles all the way down and into these cracks. One way of doing this is to increase the viscosity of the fracturing fluid, i.e., to make the fracturing fluid thicker. This is what Kern did by using a compound having boron such as boric acid. The boron, as borate ions, in the boric acid would attach to different molecules of guar to make the fluid thicker. The more boron as borate ions is present, the more guar molecules are attached together and the thicker the fluid.
Therefore, fluid viscosity and fluid stability are dictated by the concentrations of the polysaccharide and of borate ion. For a given borate ion concentration, increasing the amount of guar will increase fluid viscosity and will increase the amount of time for which that fluid can be held at a given temperature without significant loss of viscosity. The fluid is much more sensitive to the concentration of borate ion. If there is too little borate ion, no significant viscosity will be developed and the fluid will not be useful. If there is too much borate, the fluid becomes over-crosslinked, i.e., the guar reacts with the borate ions to form tight little balls of polymer in a water-thin fluid. This fluid has no effective viscosity and is not able to support and transport proppant. Thus, the borate ion concentration must be controlled within a very narrow window in order to have a viable fluid for hydraulic fracturing applications.
Another characteristic of the boron cross-linked guar is that it is sensitive to pH. As noted above, the bonds or ties between the boron and the guar molecule are in equilibrium. pH is a measure of how acidic or basic a fluid is. As the pH is made more basic, the boron is more inclined to attach itself to a guar molecule. As it becomes more acidic, the boron material tends to stay in the form of boric acid and does not attach itself to the guar molecule. A pH of 7 is said to be neutral. A pH greater than 7 is said to be basic and it goes all the way up to a pH of 14. A pH less than 7 is said to be acidic and extends to a pH of 1 (and even into negative numbers).
Though boron may be supplied in a variety of ways, it must be present as borate ions in order to serve as a crosslinker for polysaccharides, e.g., guar. According to D. J. Doonan and L. D. Lower ("Boron Compounds (Oxide, Acids, Borates)", in Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 4, p. 67-110, 3rd ed., 1978), boric acid, borate ion and polyions containing various amounts of boron, oxygen, and hydroxyl groups exist in dynamic equilibrium where the percentage of each of the species present is dictated mainly by the pH of the solution. Borate ion begins to dominate the other boron species present in the fluid at a pH of approximately 9.5 and exceeds 95% of total boron species present at a pH of about 11.5. According to B. R. Sanderson ("Coordination Compounds of Boric Acid" in Mellor's Comprehensive Inorganic Chemistry, p. 721-764, ca. 1975), boron species (including borate ions and boric acid among others) react with di- and poly-hydroxyl compounds having a cis-hydroxyl pair to form complexes which are in rapid equilibrium with the uncomplexed boron species and the cis-hydroxyl compounds. The relative amounts of the complexed and free materials are provided by the equilibrium constants for the specific systems. The equilibrium constants for borate ion is several orders of magnitude larger (typically by factors of 10.sup.4 to 10.sup.10) than the equilibrium constant for boric acid with the same cis-hydroxyl compound. For all practical purposes, borate ions form complexes (i.e., can serve to crosslink polysaccharides), while boric acid does not. Therefore, in order to have a useable crosslinked polysaccharide fluid with the minimum boron content, most of the boron must be present as borate ions which requires a pH of at least about 8.5, preferably at least about 9.5. For example, in Kern in order to crosslink the guar with the boron, the pH was made basic, preferably between pH 8.5 to 12, when the boron compound is added to the hydrated guar. He adjusted the pH of the fluid by adding a base, specifically sodium hydroxide.
Unless a fluid is adequately buffered, pH will decrease with increasing temperature. (This is generally true of buffered fluids as well, it is just that the change in pH is much less severe for buffered fluids.) For a solution prepared with sodium hydroxide with a room temperature pH reading of 12, raising the temperature by 100.degree. F. will decrease the pH by more than 1 unit. Since we are mostly interested in use above 200.degree. F., the change in pH from the time it is prepared until it is at the maximum temperature in the reservoir will be greater than 1 pH unit, and could easily be as much as 2 or more pH units. This change is sufficient to cut the effective concentration of borate ions in half if we start at a pH of 11.5 or higher. It is enough to reduce the effective concentration of borate ions by a factor of 4 if we start at a pH of 10.5. The pH of the fluid prepared at the surface and the boron level must be specially controlled to provide the optimum fluid performance in the down hole environment.
One of the problems with boron acting as a tying agent or a cross-linking agent that ties two molecules of guar together is that the ability to form such attachments or bonds is sensitive to temperature. The bonds or the ties between the boron and the guar molecules is not a permanent bond, but is said to be in equilibrium. This means that it can undo itself and reattach itself at the same or another point of the guar molecule or with another molecule of guar. Further, the guar is like a coil of material. As it is heated, it relaxes and can extend itself. At some point, it becomes a little more difficult for the boron that is attached to one molecule of guar to find another molecule of guar to attach to.
One way of getting around this problem is by adding additional boron, batchwise or on-the-fly, to the system to increase the chances of the boron as a borate ion attaching two guar molecules together and maintaining the viscosity of the fracturing fluid to a point that it can still carry the sand particles into the cracks in the formation. However, adding additional boron to the fracturing fluid at the surface quickly and greatly increases the viscosity or thickness of the fluid and makes it very difficult to pump down the well bore requiring additional horsepower, thereby increasing the cost of the job. Further, using these higher pressures to pump the viscous fluid down the well bore increases the friction of the fracturing fluid against the well bore wall causing damage either to the well bore wall or to the fracturing fluid itself by being pulled apart as it is going down the well bore wall as a result of the friction. Accordingly, there was a need to delay the increase in viscosity needed down in the well bore so that the increased viscosity was not seen at the surface while the fluid was being pumped down into the well bore. Preferably, the increased viscosity happened close to the point where fracturing was to occur or at least to the point at which the temperature was sufficiently high that additional boron was needed to be released to maintain the viscosity of the fluid so that the sand would not fall out of the fracturing fluid. If the sand particles could not be sustained by the fracturing fluid, then they could not be placed within the cracks in the formation formed by the fracturing operation to maintain the cracks open after the pressure was released.
One way of delaying the cross-linking between the guar molecule and the boron was to use a slowly dissolving material. The material could be a slowly dissolving base (see, e.g., U.S. Pat. No. 3,974,077 to Free; used magnesium oxide as base or a wax-encapsulated base) or a slowly dissolving boron-containing material (see e.g., U.S. Pat. No. 4,619,776 to Mondshine). For example, with a slowly dissolving base, the pH would not be increased until the materials were further down the well bore. Accordingly, additional boron-containing material could be added at the surface but would not increase the viscosity or thickness of the fracturing fluid until the slow dissolving base dissolved and increased the pH to become basic and therefore encourage the boron containing material to crosslink or attach itself to the guar molecules. Alternatively, a slow dissolving boron-containing material could be used. The pH of the fracturing fluid could be made basic but since the boron-containing material was slowly dissolving, it would not increase the viscosity or thickness of the fracturing fluid until it got further down into the well bore and the boron-containing material dissolved. A modification of this was to use a solid boron-containing material that was coated or encapsulated in a material that would slowly dissolve, erode or melt away down in the formation as the temperature increased.(see, e.g., U.S. Pat. No. 3,898,165 to Ely which discloses the use of borax particles encapsulated by a paraffin wax for high temperature stability in fracturing fluids). Either way, a delay in the cross-linking or attachment of the borate ions provided by the boron-containing materials to the guar molecules was effected.
As noted above, the attachment of the boron materials to the guar molecules is also temperature sensitive. Further, the guar molecules, which are fairly long molecules, are all coiled up at lower temperatures such as temperatures experienced at the surface compared to temperatures experienced down in the well formation. Accordingly, at the surface there are fewer locations for the borate ions to attach themselves to guar molecules because the guar molecules are coiled up. Therefore, if additional boron material is added at the surface which provides borate ions in excess of that which will bind up the available sites on the guar molecules, then this excess would be available down in the well bore. In U.S. Pat. No. 3,215,634 to Walker, issued Nov. 2, 1965, he did just that, but added a polyhydric alcohol containing from 2 to 5 carbon atoms, for example, glycerol and ethylene glycol, to stabilize the fluid and reduce the fluid's sensitivity to temperature change. A polyhydric alcohol is an alcohol that has more than one "--OH" group which is called a hydroxyl group. Guar molecules have hydroxyl groups in them. Guar is like a long chain in which the various links of the chain are various molecules of simple sugars or sugar residues, i.e., what remains when sugar molecules are pulled apart (hereinafter referred to as "simple sugars") attached to each other. Simple sugars or sugar residues are also called monosaccharides. Certain of the simple sugars in the guar chain have pairs of hydroxyl groups that are in a "cis" orientation, i.e., parallel to each other and extend in the same direction. When a boron material attaches itself to a guar molecule, it attaches itself at a point where these two hydroxyl groups are located. Walker recognized this fact and utilized it to his advantage. He used polyhydric alcohols which had at least two hydroxyl groups in the same type of position that are presented in the guar molecule. Accordingly, since these hydroxyl groups are not tucked away within the coils of the guar molecule, they are readily accessible for binding or attaching to the excess boron present as borate ions that is available. This also keeps the guar molecules from becoming overly attached to each other through the borate ions such that the attached or cross-linked guar molecules tend to precipitate or fall out of solution. As noted above, the bonds or attachments between the boron and these hydroxyl groups are in equilibrium and attach and reattach to the same or other pairs of cis-hydroxyl groups. Accordingly, as the fracturing fluid continues down its journey within the well bore and the temperature increases, the guar molecules tend to uncoil exposing more hydroxyl groups. Since the borate ions attach and reattach themselves to these pairs of hydroxyl groups on the polyhydric alcohols or on the guar molecules, and the guar molecules are present in a greater concentration than the polyhydric alcohols, the boron tends to seek or finds more of the pairs of hydroxyl groups on the guar molecules to attach itself to. Accordingly, the viscosity or thickness of the fracturing fluid is maintained at a place down within the well bore by using the "reserve" of boron attached to the alcohol, rather than increasing the viscosity or thickness of the fluid at the surface to a point where it is difficult to pump or can damage the fluid or well bore on the way down.
It appears that the selection of the cis-hydroxyl alcohol and the amount to be used are important. In Walker (U.S. Pat. No. 3,215,634), glycerol and ethylene glycol are the preferred alcohols for this purpose. The general use of polyhydric alcohols as stabilizers for aqueous solutions of polysaccharides has been taught by Foster (U.S. Pat. No. 3,346,556). Sorbitol was one of the preferred stabilizers in this work. On the other hand, Freidman (U.S. Pat. No. 3,800,872) teaches that the addition of cis-hydroxyl alcohol containing compounds such as glycerol can result in formation of sufficient borate-glycerol complex to destroy the crosslinked structure of the fluid. Please note that glycerol is one of the preferred stabilizers in Walker.
As the foregoing illustrates, borate-crosslinked polysaccharide-based fluids for hydraulic fracturing have been used in the industry for more than 30 years. Having to deal with deeper wells has brought the temperature and pH sensitivity of such systems to the forefront as a problem needing to be addressed. Initial borate fluids were designed for use at temperatures below about 200.degree. F. and developments concentrated on providing a fluid which was stable under the desired end use conditions. For example, the prior art discussed above for the most part was concerned with reservoir temperatures of about 150.degree. F. See, e.g., Kern (concerned with fluid stability at about 150.degree. F.), Wyant (U.S. Pat. No. 3,079,332; about 150.degree. F.), and Walker (U.S. Pat. No. 3,215,634; maximum temperature tested was 141.degree. F.). Since then, fluid systems been improved to allow use at temperatures above 200.degree. F.
Dawson (U.S. Pat. Nos. 5,082,579; 5,145,590 (up to about 300.degree. F.); U.S. Pat. Nos. 5,160,643), Sharif (U.S. 5,160,445; 5,310,489) and Harris (U.S. Pat. No. 5,372,732) teach the use of deliberately pre-formed organo-boron complexes as delay agents for borate crosslinked polysaccharide fluids for use above about 200.degree. F.
As earlier noted, Mondshine (U.S. Pat. No. 4,619,776) teaches the use of slowly soluble inorganic boron sources to control the rate of boron availability. Mondshine also teaches the use of a combination of rapidly soluble boron source and slowly soluble boron source wherein the rapidly soluble boron source can provide some initial minimum viscosity with the slowly soluble boron source providing a "reserve" source of soluble boron to enhance the thermal stability of such fracturing fluids." This fluid was designed for use up to about 275.degree. F.
All of the above methods of controlling delay time of borate crosslinked polysaccharide fluids for hydraulic fracturing applications suffer from the limitation that changing the delay time requires changes in the concentration of boron and/or of the pH of the fluid. These are the two main factors governing fluid stability. Therefore, changes in delay time require significant modification in fluid composition in order to provide the same fluid stability. As a result, it is difficult and/or potentially job threatening to modify the delay time of borate fluids on location in general, let alone during the course of the actual treatment when such optimization may be desirable. Further, the pre-formed organo-boron complexes and the fracturing fluids incorporating them are very sensitive and precise formulations. In such processes, it is therefore very important to perform comprehensive pre-job testing for successful fracturing.
Progress has been made both in improving fluid stability which is important at all temperatures as well as in methodology to control the onset of cross-linking (delay time) so that the fluid would develop the right viscosity for successful utilization at the right time in the life of the fluid. These aspects of fluid chemistry are interrelated. As a result, existing methods of providing borate fluids for use above about 200.degree. F. typically compromise fluid viscosity and stability in order to obtain desired delay time.